In recent years, technology for converting thermal energy into electrical energy has been widely researched. However, it is a fact that energy conversion utilizing the thermoelectric effect is not fully utilized in such major energy conversion systems as thermal power plants and nuclear power plants.
A brief explanation of the reason for this under-utilization will now be given. FIG. 1 is a schematic representation of an energy flow inside a thermoelectric conversion element. It should be noted that this diagram is taken from Echigo's thesis titled "What Is the Problem with Thermoelectric Conversion Technology?" (Energy Shigen Gakkai, July 1995, p. 43). Additionally, the breadth of the diagram is proportional to the size of the energy flux.
As shown in FIG. 1, the majority of the heating heat flux q.sub.H is in fact expended in useless heat transfer flux q.sub.HC unrelated to thermoelectric conversion, with an effective Peltier heat q.sub.HP relegated to a supporting role. Here, if the figure of merit is z and the high-temperature edge temperature is T.sub.H, with z.multidot.T.sub.H.apprxeq.1, then the heat transfer flux q.sub.HC can become approximately twice the Peltier heat q.sub.HP and moreover, of the true Peltier heat from which the Peltier heat discharge has been subtracted (q.sub.HP -q.sub.LP), approximately half becomes Joule heat q.sub.J. Accordingly, essentially that which is converted into electric power is heat P, so the conversion efficiency is low and the temperature, which is low, becomes even lower.
If, here, for example, the temperature of the low-temperature side is T.sub.L =300.degree. C. and T.sub.H =1300.degree. C., then the maximum efficiency is 13.7%. Moreover, even assuming a dramatic advance in materials such that z.multidot.T.sub.H.apprxeq.2, the maximum efficiency will still be no more than 22.0%. This point is the biggest problem with thermal power generation, and the inability of thermal power alone to compete with other power generating facilities such as gas turbines and fuel cells is the largest factor impeding its technological development.
FIG. 2 shows a figure of merit for various types of thermoelectric material. The vertical axis represents the figure of merit and the horizontal axis represents the temperature. It can be understood from FIG. 2 that the energy conversion rate of thermoelectric material is at most approximately 10%. Accordingly, further research and development is required for the development of thermoelectric element material.
FIG. 3 shows the temperature dependency of a coefficient of thermal conductivity of a packed skutterudite alloy. As shown in FIG. 3, a packed skutterudite alloy having such compositions as LaFe.sub.3 CoSb.sub.12 and CeFe.sub.3 CoSb.sub.12 has a coefficient of thermal conductivity that is an order of magnitude smaller than the coefficient of thermal conductivity of a binary skutterudite alloy CoSb.sub.3 having superior electrical characteristics, and at ordinary temperature is about the same as glass-type silica (SiO.sub.2).
FIG. 4 shows the temperature dependency of a dimensionless figure of merit ZT in a substantially optimized sample of packed skutterudite alloy. The black dots in FIG. 4 represent the above-described temperature dependency, assuming a single parabolic band and acoustic phonon scattering and calculated on the basis of carrier density at measured room temperature. Additionally, the measured value at high temperature of the coefficient of thermal conductivity is also reflected in these calculations.
Here, there is no theoretical upper limit to the value of the dimensionless figure of merit ZT and there are possible candidates whose dimensionless figure of merit ZT is about 3-4. However, such a material has not at present been found.
It should be noted that FIG. 3 and FIG. 4 were cited in "Parity", Vol.12 No. 10 1997-10.
At the same time, power generation systems combining a gas turbine and a steam turbine have been studied as a technology for converting thermal energy into electrical energy, and a variety of combined cycle power generating systems with improved power generating efficiency have been proposed.
In a steam turbine, the maximum temperature (the temperature at the critical pressure of water) is 566.degree. C. and the combustion gas temperature is approximately 1,500.degree. C. or more, so as a heat engine enthalpy is not utilized in the range 1,500.degree. C.-566.degree. C. Accordingly, combined cycle power generating systems can be said to be systems designed with this factor, which reduces power generating efficiency, in mind.
It should be noted that the shaft configuration of a combined cycle power generating system (also called simply a "combined system") may be either a single-shaft type, in which one gas turbine and one steam turbine are linked along the same shaft in the exhaust heat recovery cycle, or a multiple-shaft type, in which the gas turbines and the steam turbines are operated using separate shafts, with the type selected depending on the purpose, operating methods and installation conditions.
FIG. 5 is a diagram for illustrating the combined cycle power generating system of the conventional exhaust heat recovery type. As shown in FIG. 5, this conventional exhaust heat recovery type combined cycle power generating system comprises a gas turbine 19, an exhaust heat recovery boiler 21 connected to the gas turbine 19, a steam turbine 23 connected to the exhaust heat recovery boiler 21, a condenser 25 connected to the steam turbine 23, and a feed water pump 27 connected between the condenser 25 and the exhaust heat recovery boiler 21.
Here, as shown on page 126 of the Electrical Engineering Handbook, the higher the temperature of the gas turbine 19 the greater the thermal efficiency of the combined system overall.
In the system shown in FIG. 5, the gas turbine 19 is turned using the energy of gas heated to a high temperature of approximately 1,500.degree. C. and at the same time the steam turbine, too, can be operated by using the exhaust gas to generate steam, so overall the power generating efficiency increases.
Additionally, as a steam turbine engine designed for improved thermoelectric conversion efficiency, a system using a reheating cycle in which all steam is removed from an intermediate drop of the turbine, reheated and once again sent to the turbine has conventionally been proposed. FIG. 6 is a heat balance diagram of an actual thermal power plant system utilizing this reheating cycle.
As shown in FIG. 6, after the high-temperature, high-pressure steam generated at the boiler 29 rotates a high-pressure turbine 31 it is once again heated by a reheating unit and fed to a medium-pressure turbine 33. Additionally, some of the steam forms as water on the blades of the turbine. This steam and water is cooled by the condenser 35 and returned to low-temperature, low-pressure water and fed to the feed water heater 37. The water supplied from the condenser 35 is reheated by the feed water heater 37, compressed once more by the feed water pump 39 and supplied to the boiler 29. It should be noted that at present the power generating efficiency of the thermal power plant is approximately 39%.
Ways to improve the technology for converting thermal energy into electrical energy have been considered from a variety of angles as described above. A description will now be given of the thermoelectric conversion technology utilized in energy conversion systems in thermal power plants.
FIG. 7 is a diagram for illustrating the conventional thermoelectric conversion element. As shown in FIG. 7, carbon 3 as the N-type semiconductor element and B.sub.4 C (boron carbide) as the P-type semiconductor element are both joined to a heat-collecting metallic plate 1 made from W or Mo as the P-N junction electrode, with electrodes 7 positioned at the ends of the carbon 3 and boron carbide 5 opposite the heat-collecting metallic plate 1. An output voltage is then obtained from the potential difference generated at the two electrodes 7. It should be noted that water pipes 9 penetrate through-holes formed in each of the two electrodes 7.
According to this type of thermoelectric conversion element, if for example the temperature of the heat-collecting metallic plate 1 is 1,500.degree. C. and the temperature of the electrodes 7 is 30.degree. C., a maximum 8% thermal efficiency can be obtained. If the temperature of the heat-collecting metallic plate 1 is 1,500.degree. C. and the temperature of the electrodes 7 is 600.degree. C., then a maximum 4% thermal efficiency can be obtained.
FIG. 8 shows the structure of a conventional thermal power plant in which the thermoelectric conversion element shown in FIG. 7 has been included. As shown by the portion of FIG. 8 covered by slanted lines, the thermoelectric conversion elements 13 are installed in columns around the combustion chamber of the boiler 11 as shown in FIG. 7. The thermoelectric conversion elements 13 are arranged so that the heat-collecting metallic plates face the combustion chamber, and at the same time the water that has been supplied to the boiler 11 via the feed water pipe 17 circulates through the water pipes 9 shown in FIG. 7 and steam is generated. Then, this steam is supplied from an output port 15 of the boiler 11 to the steam turbine. Combustion gasses produced in the boiler 11 are discharged from a smokestack 12.